1. Field of the Invention
This invention relates to an electric signal identification system, and more particularly to a single-frequency or multi-frequency signal identification system for use in telephone switching, data switching, etc.
2. The Prior Art
Recently, a demand for domestic and international telephone communication has remarkedly increased. To meet with the demand, countermeasures such as introduction of electronic switching apparatus and increases of other equipments are now being taken. In signalling systems for telephone switching, a domestic common channel signalling system and a new signalling system of the type providing a separate digital transmission line for signal transmission, which is called No. 6 signalling system, have now been tested for commercialization. However, in most cases, use is made of a signalling system employing in-band or out-band voice frequency signals.
Voice-frequency signals for telephone switching are divided into a supervisory signal for supervising a call switching process and an address signal for a call switching direction. These signals are each composed of a single-frequency or multi-frequency signals in a voice frequency band. In international telephone switching, for example, in the case of the No. 5 signalling system, the supervisory signal is composed of one or both of voice-frequency signals of 2400Hz and 2600Hz. The address signal is refered to as an MF signal as employed in domestic telephone systems and is composed of signals of six frequencies which are spaced apart at intervals of 200Hz in the frequency band range from 700Hz to 1700Hz. By combining pairs of the six frequencies, a digit, a start of message signal and an end of message signal are indicated.
One method that has been usually employed for the identification of such voice signals is to extract a signal of a desired frequency from an input signal through a filter and then to convert the extracted signal into a DC level for identification. In this method, however, the filter occupies, due to its construction, a large part of the identification device of such a low-frequency signal as a telephone signal and, further, there arise problems peculiar to an analog circuit such as a characteristic change due to ageing, etc.